select purchase_orders.purchase_order_id, order_date, department_id
from purchase_orders
join purchase_order_lines
on purchase_orders.purchase_order_id = purchase_order_lines.purchase_order_id
where not exists (select purchase_orders.purchase_order_id = purchase_order_lines.purchase_order_id);
我试图确保它仅显示不属于采购订单行的采购订单。我不断收到错误消息，我不确定如何使其工作。任何帮助都会很棒。